Edwardsiella ictaluri, a Gram-negative rod, is the etiological agent of enteric septicemia (ESC) of catfish (Hawke, 1979). ESC is one of the most prevalent diseases of farmed channel catfish (Ictalurus punctatus), which is the largest aquaculture industry in the United States. ESC occurs in two forms: acute enteric septicemia and a chronic encephalitis (Shotts et al., 1986).
Although oxytetracycline, sulphadimethoxine/ormetoprim, and florfenicol are approved antibiotics in the United State for treatment of ESC in food fish, there are reports of bacterial resistance to antibiotics (Plumb et al., 1995; Smith et al., 1994). Also, anorexia is one of the earliest clinical signs of ESC. Therefore, these antibiotics are more effective in limiting the spread of an outbreak than in treating the disease. An alternative strategy for preventing ESC would be beneficial to the industry.
Vaccination is an efficient method for prevention and control of ESC. Edwardsiella ictaluri is a candidate for the development of a live attenuated vaccine due to the antigenic homogeneity of all Edwardsiella ictaluri isolates (Bertolini et al., 1990; Plumb and Vinitnantharat, 1989). There is a commercial vaccine for ESC (RE-33, commercial brand name AQUAVAC-ESC), which is developed by serial passage in increasing concentration of rifampicin resistance. Fry as young as 7 d post-hatch, developed an immune response when vaccinated with Aqua AQUAVAC-ESC (Klesius and Shoemaker, 1999). However, this vaccine is rifampicin resistant and antibiotic resistance is not a desired trait in a live attenuated vaccine. In addition, the genetic alteration or reason for attenuation in AQUAVAC-ESC is not unknown completely (Klesius and Shoemaker, 1997). Despite the availability of AQUAVAC-ESC and several previous attempts to develop an immersion-oral Edwardsiella ictaluri vaccine, however, ESC is still one of the most prevalent diseases threat to the catfish industry. Therefore, there is an urgent need for an effective vaccine that can prevent ESC and can be safely delivered to catfish fry before their release into production ponds.